Problem: Which of these numbers is composite? ${2,\ 47,\ 51,\ 61,\ 79}$
Explanation: A composite number is a number that has more than two factors (including 1 and itself). 2, 47, 61, and 79 each have only two factors. The factors of 51 are 1, 3, 17, and 51. Thus, 51 is the composite number.